In an electrophotographic device which utilizes an electrophotographic process, such as a copier or a laser printer, images formed on a photosensitive material by an electrophotographic process are transferred onto transfer sheets in the form of toner images. After fixing the transferred images on the sheets with an image fixing unit, the sheets are discharged from the device. The image fixing unit comprises an image fixing roller with a built-in heater, and a pressure roller pressed against the image fixing roller. Transfer sheets carrying toner images are passed through a nip portion between the rollers to melt and fix toner to the transfer sheets by heating and pressing. Then, the transfer sheets are discharged from the device by means of sheet feed rolling elements such as sheet discharge rollers. In some image fixing units, an image fixing member such as an image fixing belt is used instead of the image fixing roller.
FIG. 6 shows specific sheet feed rolling elements. FIG. 6 schematically shows an image fixing unit including an image fixing roller. The image fixing unit shown in FIG. 6 comprises the image fixing roller 22, which has a built-in heater 21, and a pressure roller 23 which is rotated by the image fixing roller with a transfer sheet sandwiched therebetween. A transfer sheet 25 fed from a transfer station, not shown, by a conveyor belt 24 and carrying unfixed toner images is guided by an image fixing unit entrance guide 26 and inserted into a nip portion between the image fixing roller 22 and the pressure roller 23. The rollers 22 and 23 sandwich the sheet, thereby feeding the sheet while fixing the toner images to the sheet. After the images are fixed, the leading end of the sheet 25 is peeled off from the image fixing roller 22 by the tip of a separation claw 27 located downstream of the nip portion of the image fixing roller 22. Once the sheet is peeled off from the image fixing roller 22, the sheet is fed through a feed path where the sheet passes between a sheet discharge roller 29, which is being driven, and a sheet discharge roller 28, which is pressed against and rotated by the roller 29, and is discharged from the device.
FIG. 7 shows a color laser printer in which after a transfer sheet carrying unfixed toner images passes between an image fixing roller 30 and a pressure roller 31, the sheet is peeled off from the image fixing roller 30 by a separation claw 32. The sheet then passes between the sheet discharge rollers 33 and 34, between a pair of intermediate guide rollers 35, and then between a driven roller 36 and an eject roller 37, and is discharged from the device. In FIG. 7, numeral 32a designates a separation claw in contact with the pressure roller 31. Numeral 38 in FIG. 7 indicates a laser unit; 39 is a toner storage; 40 is a photosensitive drum for forming electrostatic latent images; 41 is a transfer unit; 42 is a paper (sheet) storage cassette; and 43 is an image fixing unit.
Conventional sheet feed rolling elements used in the above-described image forming devices, such as sheet discharge rollers, intermediate guide rollers and eject rollers, are shown in FIGS. 8(a), 8(b) and 8(c), which are perspective views of the respective sheet feed rolling elements. The sheet feed rolling elements shown in FIGS. 8(a) and 8(b) are used as sheet discharge rollers or intermediate rollers, and each comprise a roller portion 51 and has support shafts 52 as rotary shafts at both ends of the roller portion 51, or a shaft hole 53. The sheet feed rolling element shown in FIG. 8(c) is used as an eject roller, and comprises a roller portion 51 and geared support shafts 54 at both ends of the roller portion 51. These sheet feed rolling elements are formed by injection molding a melt type fluororesin such as a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-ethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or tetrafluoroethylene-hexafluoropropylene copolymer (FEP) (see JP 9-315615A).
Molds for forming such rolling elements have a parting line that are provided at a position corresponding to one end of the roller portion 51 or spaced not more than 2 mm from one end of the roller portion 51 so that the parting line mark 51a, which is in the shape of a rib, does not look conspicuous and so that the sheet feed rolling element can be easily removed from the mold (see JP 2000-355434A). The position spaced not more than 2 mm from one end of the roller portion 51 is determined based on the size of the curved chamfer formed at each shoulder (edge) of the roller portion 51. For example, if the curved chamfer has a radius of 1 mm, the parting line (hereinafter abbreviated to “PL”) is provided at the position spaced by 1 mm from the end of the roller portion.
But in this case, a circumferential parting line mark (hereinafter abbreviated to “PL mark”) in the shape of a narrow strip is formed on the sheet sliding contact surface on the outer periphery of the sheet feed rolling element, and this mark is transferred to transfer sheets in the shape of a narrow strip. Thus, it is necessary to remove the PL mark by subjecting the sheet feed rolling element to post-treatment such as barreling or shot blasting. Such post-treatment is time-consuming and troublesome.
In order to avoid this problem, it is proposed to provide the PL at a position corresponding to the longitudinal central portion of the sheet sliding contact surface and to form the roller portion such that the sheet sliding contact surface has an outer diameter at its longitudinal central portion that is smaller than its outer diameter at both ends (see JP 2000-355434A).
There is also known a sheet feed rolling element for use in a color electrophotographic device which includes a film-like contact portion to be brought into contact with transfer sheets in order to reduce the contact pressure on transfer sheets, thereby improving the quality of images, and a mold for injection molding used to produce such a sheet feed rolling element (see JP 2004-189485A and Patent document 4: JP 2004-314310A).
With the sheet feed rolling element disclosed in JP 2000-355434A, a surface sink mark (recess) that forms during injection molding is used in order that the sheet sliding contact surface has an outer diameter at its longitudinal central portion that is smaller than its outer diameter at both ends. But it may be difficult to prevent the PL mark from protruding from the sheet sliding contact surface using the surface sink mark, because the sheet sliding contact surface of the sheet feed rolling element has a small outer diameter. With a sheet feed rolling element having a film-like sheet sliding contact surface too, it is difficult to prevent the PL mark from protruding from the sheet sliding contact surface using the surface sink mark, because the outer peripheral portion has a small wall thickness.
Under these circumstances, in an attempt to prevent the formation of a PL mark in the shape of a streak, the inventors of the present invention manufactured a sheet feed rolling element having a sheet sliding contact surface with a small outer diameter and shoulders on both sides which are formed by curved chamfers, with PL located where one of the curved chamfer is formed, and with the curved chamfers having an outer diameter slightly smaller than the outer diameter of the sheet sliding contact surface. But on the sheet sliding contact surface of the sheet feed rolling element actually formed by injection molding, a circumferential bulge formed near PL having a maximum radial dimension of 50 μm and a maximum axial dimension of 100 μm. Since this bulge causes streaks on transfer sheets, it is necessary to remove the bulge by subjecting the sheet feed rolling element to post-working such as barreling or shot blasting as in the conventional arrangement. Such post-working is troublesome.
Similar problems are observed in a sheet feed rolling element formed using the mold for injection molding as disclosed in Patent document 4, and having a film-like portion having the sheet sliding contact surface and a roller portion having shoulders on both sides which are formed by curved chamfers.